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| United States Patent |
5,286,560
|
|
Fishkis
, et al.
|
February 15, 1994
|
Method for increasing the wettability of aluminum metal to alumina
containing fibers
Abstract
An aluminum alloy matrix composite containing alumina or aluminum silicate
reinforcement whereby said composite material exhibits wetting between
said reinforcement and said aluminum alloy. The composite comprises (a)
alumina or aluminosilicate reinforcement; (b) a matrix of aluminum or
aluminum alloy; and (c) an intelayer of mixed oxides of aluminum and boron
wherein the interlayer of mixed oxides of aluminum and boron includes but
is not limited to the following oxides, B.sub.2 O.sub.3, Al.sub.2 O.sub.3,
2Al.sub.2 O.sub.3.B.sub.2 O.sub.3 and 9Al.sub.2 O.sub.3 2B.sub.2 O.sub.3
at the interface between said reinforcement and said matrix.
| Inventors:
|
Fishkis; Maya (Plum Boro, PA);
Misra; Chanakya (Plum Boro, PA);
Wefers; Karl (Apollo, PA)
|
| Assignee:
|
Aluminum Company of America (Pittsburgh, PA)
|
| Appl. No.:
|
968473 |
| Filed:
|
October 29, 1992 |
| Current U.S. Class: |
428/357; 164/97; 423/276; 423/277; 428/344; 428/375; 428/416; 428/432 |
| Intern'l Class: |
C22C 001/09; C22C 001/10; B22D 019/14; B32B 018/00 |
| Field of Search: |
428/344,375,357,416,432
164/91
423/276,277
|
References Cited
U.S. Patent Documents
| 4012204 | Mar., 1977 | Riewald et al. | 29/191.
|
| 4053011 | Oct., 1977 | Riewald et al. | 164/97.
|
| 4232091 | Nov., 1980 | Grimshaw et al. | 428/472.
|
| 4450207 | May., 1984 | Donomoto et al. | 428/614.
|
| 4687043 | Aug., 1987 | Weiss et al. | 164/97.
|
| Foreign Patent Documents |
| 2-101127 | Apr., 1990 | JP.
| |
Primary Examiner: Ryan; Patrick J.
Assistant Examiner: Weisberger; Richard P.
Attorney, Agent or Firm: Pearce-Smith; David W.
Parent Case Text
this application is a continuation of application Ser. No. 07/674,120 filed
Mar. 25, 1991, now abandoned.
Claims
What is claimed is:
1. An aluminum alloy matrix composite containing alumina or aluminum
silicate reinforcement whereby said composite material exhibits wetting
between said reinforcement and said aluminum alloy, said composite
comprising:
(a) alumina or aluminosilicate reinforcement;
(b) a matrix of aluminum or aluminum alloy; and
(c) an interlayer of mixed oxides of aluminum and boron at the interface
between said reinforcement and said matrix wherein the interlayer of mixed
oxides of aluminum and born includes but is not limited to the following
oxides, B.sub.2 O, Al.sub.2 O.sub.3, 2Al.sub.2 O.sub.3.B.sub.2 O.sub.3 and
9Al.sub.2 O.sub.3 2B.sub.2 O.sub.3.
2. The aluminum alloy matrix composite of claim 1 in which said
reinforcement is alumina fibers.
3. The aluminum alloy matrix composite of claim 1 in which said
reinforcement is alumina particles.
4. The aluminum alloy matrix composite of claim 1 in which said interlayer
of mixed oxides of aluminum and boron are formed by heating fibers coated
with material selected from the group consisting of boron, boron oxide and
boron oxide precursors above 800.degree. C. for at least 1 minute prior to
coating with said matrix material.
5. The aluminum alloy matrix composite of claim 1 in which said interlayer
of mixed oxides of aluminum and boron are formed by heating fibers coated
with material selected from the group consisting of boron, boron oxide and
ammonium pentaborate, ammonium diborate and orthoboric acid above
800.degree. C. for at least 1 minute prior to coating with said matrix
material.
6. The aluminum alloy matrix composite of claim 1 in which said
reinforcement is selected from the group consisting of alumina,
cordierite, mullite, aluminum silicate or polycrystalline aluminum.
7. The aluminum alloy matrix composite of claim 1 in which said
reinforcement is selected from the group consisting of particles, planar
sheets, fibers, whiskers and combinations thereof.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to aluminum/alumina composite materials and
to methods for increasing the wettability of alumina fibers by molten
aluminum or an aluminum alloy. More particularly, the present invention
relates to improving the bonding at the interfaces between alumina fibers
and aluminum metal.
2. Description of the Prior Art
The main difficulty encountered in forming alumina/aluminum metal matrix
composites (MMCs) is that molten aluminum and its alloys do not readily
wet alumina. The wetting of the fibers by molten metal is critical to the
formation of useful MMCs.
The function of the fibers in composites is to increase the strength and
the fracture toughness. These two functions impose contradictory
requirements on the alumina/aluminum interface. High strength is achieved
through good load transfer from the matrix to the fibers and it requires
strong bonding at the interface. High fracture toughness is achieved
through crack energy dissipation.
In the past, it has been proposed to manufacture composite materials which
comprise reinforcing-alumina fibers enclosed in an aluminum metal matrix
by impregnating a suitable assembly of fibers with the molten metal.
Alumina fibers do not exhibit strong reactivity with aluminum alloys and
therefore they can be used as a compatible reinforcement material. In
addition, alumina is very refractory (melting temperature is 1999.degree.
C.-2032.degree. C.) and are capable of withstanding processing
temperatures of molten aluminum.
It is desired that the alumina fibers be impregnated with the molten
aluminum metal either through capillary action in which the fibers are
partially or wholly immersed in the molten metal which can be aided by
vacuum action in which the fibers are enclosed in an evacuated chamber and
the molten metal is admitted into the chamber.
Incomplete wetting of the alumina fibers and the aluminum metal matrix
creates voids within the resultant composite material which, in turn,
weakens the composite. Then too, even if acceptable wetting is achieved,
sufficient bond strength at the interface between the fiber and the metals
is desired to obtain high strength. Additional difficulties may be
encountered during subsequent welding or brazing of the composite
materials. Localized melting of the metal matrix composite during welding
or brazing operations may cause a corresponding localized de-wetting of
the reinforcing fiber. This, in turn, leads to porosity in the region of
the weld or braze. Generally speaking, ineffective wetting and/or
ineffective bonding at the interface between the composite layers will
degrade the composite's properties.
Additionally, the impregnation of the fibers with the molten metal can take
a long time, thereby possibly causing deleterious interaction between the
fibers and the metal.
Various prior art attempts to address these problems are known. However,
the prior art methods each suffer from one or more serious drawbacks
making such methods less than entirely suitable for their intended
purposes.
For example, pressure infiltration or squeeze-casting, forcing the molten
aluminum into the fibers by employing high pressure, is a common prior art
technique. Grimshaw et al U.S. Pat. No. 4,232,091, issued Nov. 4, 1980,
describes applying at least 75 kilograms per square centimeter to overcome
the surface tension between alumina fibers and molten aluminum or alloys
of aluminum, thereby hoping to assure penetration. Donomoto et al U.S.
Pat. No. 4,450,207, issued May 22, 1984, found, furthermore, that
pressurization at approximately 1,000 kilograms per square centimeter was
required in order to infiltrate molten aluminum matrix metals (even having
from 0.5 to 4.5 weight percent magnesium for optimum bonding
characteristics) into interstices of the reinforcing alumina fibers. In
practice, these processes have tended to cause channeling in the mold and
thereby do not often assure optimum contact between a metal and the
fibers.
Additionally, lithium doping of aluminum and aluminum alloys has been
proposed to increase the wetting of the alloy on the alumina fibers. A
reaction occurs between the lithium and the alloy at the surface of the
fibers which surface becomes gray to black due to the formation of lithium
aluminate. For example, Riewald et al U.S. Pat. Nos. 4,012,204 and
4,053,011, issued Mar. 15, 1977, and Oct. 11, 1977, respectively describe
composite materials comprising an aluminum-lithium matrix, reinforced with
polycrystalline alumina fibers. In order to retain useful strength in the
fibers, not more than 15% of the total diameter of the fiber should react
with the lithium. Accordingly, the reaction conditions must be carefully
controlled insofar as initial lithium content, temperature, and
particularly pressure are concerned. In fact, a pressure differential of
about 2 to 14 pounds per square inch was needed to overcome the molten
metal's resistance to penetration into the alumina fibers.
As previously mentioned, substantial oxidation at the interface between
alumina fiber and aluminum metal has been found to be detrimental.
Recently, such a problem associated with the presence of oxides at the
surface of an aluminum metal layer was addressed by Weiss et al U.S. Pat.
No. 4,687,043, issued Aug. 18, 1987. Therein a zinc solder alloy was used
interfacially to protect an alumina fiber surface at the interface between
aluminum layers to be cast thereon.
Accordingly, it is a principal objective of the present invention to
provide a method for improved bonding of aluminum metal or alloys to
refractory fibers wherein the fibers are "wet" by the aluminum or aluminum
alloy.
Another object of the invention is to provide an alumina fiber reinforced
aluminum alloy system which exhibits good interfacial wetting that does
not require a low partial pressure of oxygen during fabrication.
A further objective of the present invention is to provide a method that
does not require an excessive pressure differential to force fiber
impregnation of molten metal.
Yet another object of the invention is to provide an alumina fiber
reinforced aluminum alloy system which exhibits good interfacial wetting
without the need to use reactive metals such as lithium as a wetting
agent.
Yet another object of the present invention is to improve the strength of
an alumina fiber/aluminum alloy MMC by increasing bond strength at the
interface.
These and other objects and advantages of the present invention will be
more fully understood and appreciated with reference to the following
description.
SUMMARY OF THE INVENTION
In accordance with the present invention, an improved process for bonding
alumina fibers and aluminum metal or alloy and effective wetting of the
fibers by the metal is unexpectedly accomplished by forming a mixed
boron-aluminum oxides (i.e., intermixture or reaction products of boron
oxides and aluminum oxide) at the interface between the fibers and the
metal or alloy.
The method of the present invention includes forming an alumina fiber
reinforced aluminum alloys. This includes the steps of (a) coating alumina
fibers with a thermally decomposable precursor of boron oxide; (b) heating
the coated fiber sufficiently to form a boron-containing oxide; and (c)
forming a composite with aluminum metal or an aluminum alloy. Thermally
decomposable precursors of boron oxide include ammonium pentaborate,
ammonium biborate and orthoboric acid. The most preferred precursor is
ammonium pentaborate.
Another aspect of the invention is an alumina fiber reinforced aluminum
matrix composite which exhibits improved bond strength and no appreciable
de-wetting of the metal and the fibers in subsequent joining. The alumina
fiber reinforced aluminum matrix composite comprises: (a) alumina fibers;
(b) matrix of aluminum or aluminum alloy metal; and (c) an interlayer of
boron-aluminum oxides at the interface between the alumina and the matrix.
The composite material exhibits improved bond strength at the
above-described interface and no appreciable de-wetting in subsequent
brazing or welding.
BRIEF DESCRIPTION OF THE DRAWINGS
Other features and advantages of the present invention will be further
described or rendered obvious in the following related description of the
preferred embodiment of the present invention which is to be considered
together with the accompanying drawings wherein like figures refer to like
parts and further wherein:
FIG. 1 is a photomicrograph of untreated alumina fibers, vapor coated with
2.5 microns of aluminum and heated above the melting point of aluminum at
100X magnification;
FIG. 2 is a photomicrograph of alumina fibers treated with ammonium
pentaborate, vapor coated with 2.5 microns of aluminum and heated above
the melting point of aluminum at 100X magnification;
FIG. 3 is an SEM of alumina fibers treated with ammonium pentaborate, vapor
coated with 2.5 microns of aluminum and heated above the melting point of
aluminum at 500X magnification;
FIG. 4 is a photomicrograph of alumina fibers coated with boron oxide and
then vapor coated with 2.5 microns of aluminum and heated above the
melting point of aluminum at 100X magnification;
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the process and the product of the present invention, the term
"boron-alumina oxide interface" may be generally described as a mixture
and reaction products of boron and aluminum oxides at the interface
between aluminum metal or alloy and alumina fibers.
The term "effective amount of boron" is used herein to describe the amount
of boron reacted at the interface of the alumina fiber and the aluminum
alloy so as to permit the desired wetting and improved bond strength of
this invention.
The term "aluminum alloy" is used herein to describe aluminum alloys having
aluminum metal as a primary constituent.
The term "liquid-phase metal" is used herein to describe all fluid and
semi-fluid phases in which the metal is not completely solidified.
Good wetting of fibers by liquid aluminum is a prerequisite for strong
bonding at the interfaces. Liquid aluminum does not wet alumina fibers
under normal processing conditions. In the processing of alumina/aluminum
(Al.sub.2 O.sub.3 /Al) composite materials, high pressure is usually
applied to force impregnation of the metal into a fiber preform. However,
even high pressure does not assure optimum contact between the aluminum
matrix and the alumina fibers.
It has been found that if alumina fibers are coated with boron oxide that
subsequent processing to create an alumina fiber/aluminum matrix composite
material yields an MMC with increased interfacial wetting. Suitable boron
amounts may be provided by way of 1.) immersion of the fibers into
saturated solutions of boron oxide precursors and then heating the coated
fibers to form boron oxide on the surface, or 2.) vapor deposition of
boron onto the surface of the fibers in a layer having thicknesses of, for
example, 300 .ANG. to 3 microns, or 3.) doping aluminum or aluminum alloy
with boron in the amount above its solubility limit prior to coating the
fibers with such alloy.
The amount of boron deposited on the surface is important in practicing the
present invention. Excessive boron amounts can leave unreacted B.sub.2
O.sub.3 on alumina surface. This would weaken the interfaces due to
melting of B.sub.2 O.sub.3. Too little boron will, of course, be
insufficient to provide the desired wetting.
As taught by Rocher et al U.S. Pat. No. 4,659,593, issued Apr. 21, 1987,
even those carbon fibers, so treated, cannot be spontaneously wet by
liquid aluminum "if placed in contact with air between the pretreatment
and the impregnation" (See Column 3, lines 17-21).
As taught by Novak et al U.S. Pat. No. 4,630,665, boron fibers such as
boron nitride are traditionally thought to be "non-wetting" with respect
to aluminum metal, unless substantial oxidation is prevented at the
interface. On the other hand, the present invention permits interfacial
oxidation to be beneficial.
As previously described, the process of the present invention may be
practiced by boron oxidized interface formed by any one of several means.
Boron oxide (B.sub.2 O.sub.3) may be formed in situ by immersing alumina
fibers in heated, saturated solutions of any thermally decomposable
precursor of boron oxide or boron oxide itself. These precursors may be,
for example, ammonium pentaborate, ammonium biborate, orthoboric acid,
etc. Preferably, ammonium pentaborate is employed.
After immersion, the coated fibers are dried at ambient temperature, for
example 75.degree. F., and thereafter heated substantially at temperatures
of from 670.degree. C. to 1450.degree. C. but preferably about
1250.degree. C. B.sub.2 O.sub.3 is known to react with Al.sub.2 O.sub.3 in
the temperature range of 788.degree. C.-1260.degree. C. (1450.degree.
F.-2300.degree. F.) to form the compounds 2Al.sub.2 O.sub.3.B.sub.2
O.sub.3 and 9Al.sub.2 O.sub.3 .multidot.2B .sub.2 O.sub.3. These compounds
are stable up to 1949.degree. C. (3540.degree. F.) and 1035.degree. C.
(1895.degree. F.), respectively. These compounds 2Al.sub.2 O.sub.3.B.sub.2
O.sub.3 and 9Al.sub.2 O.sub.3.2B.sub.2 O.sub.3 are sometimes referred to
herein as "mixed aluminum-boron oxides".
It should be noted that a heated solution of ammonium pentaborate into
which the fibers were immersed may be heated at, for example, temperatures
of between about 50.degree. C. and 90.degree. C., preferably about
75.degree. C., and for a time period needed in order to provide wetting of
the fiber surfaces by the solution. Such time ranges from between 10
seconds and 15 minutes but preferably about 1 minute is suitable.
Alternatively, boron oxide itself may be deposited on the surface of the
alumina fibers. If this process is employed the coated fibers would still
need to be heated substantially at temperatures of from about 670.degree.
C. to 1450.degree. C. so that 2Al.sub.2 O.sub.3.B.sub.2 O.sub.3 and
9Al.sub.2 O.sub.3.2B .sub.2 O.sub.3 can be formed on the surface of the
fibers prior to infiltration with the molten aluminum alloy.
A third embodiment of the present invention is to use as a matrix aluminum
alloys doped with about 1 percent by weight boron, which is above the
solubility limit. Boron would then migrate to the surface of the fibers,
oxidize and react with Al.sub.2 O.sub.3 to form 2Al.sub.2 O.sub.3.B.sub.2
O.sub.3 and 9Al.sub.2 O.sub.3.2B .sub.2 O.sub.3 on the surface and thus
promote wetting and bonding of the molten metal with the alumina fibers.
Other alloys containing the ranges of other constituents in addition to
the boron may include any wrought or cast aluminum alloy.
The fibers used in the present invention may be amorphous, single
crystalline form of alumina or aluminum silicate or a polycrystalline form
of aluminum. Aluminosilicates which may be employed include cordierite
(4(Mg,Fe)O.4Al.sub.2 O.sub.3.10SiO.sub.2) and mullite 3Al.sub.2
O.sub.3.2SiO.sub.2. In addition, other fibers which do not contain silica
may also be employed in practicing the present invention provided they
contain at least 30 wt. % Al.sub.2 O.sub.3 on their surface.
Surface treatments of this invention can be applied to the surfaces of
various alumina or aluminosilicate reinforcements which can be
consolidated with aluminum-based alloys to form composite materials. The
above-mentioned reinforcements may be particulates of any shape, alumina
whiskers, continuous fibers, woven, chopped fibers and preforms of any
shape. If particles are to be used as the reinforcement material, it is
recommended that the solution of a boron oxide precursor be agitated or
stirred to insure good wetting of the solution of the particle surfaces.
The fibers may be consolidated with aluminum-based alloys by any of the
known consolidation techniques to form composite materials. These methods
include liquid phase infiltration, squeeze casting, rheocasting,
compocasting or casting under vacuum without the use of positive pressure.
The casting may be carried out using mechanical, hydraulic, vacuum and/or
high pressure means.
The surface treatments of this invention provide higher bond strength
between the reinforcements and the aluminum alloy and thereby improve
mechanical properties, including tensile strength, of the produced
composite materials.
The novel composite materials of the present invention contain fibers of
refractory alumina or aluminosilicate, a matrix of aluminum metal or alloy
material and an interlayer of mixed aluminum-boron oxides at the interface
between the fibers and the matrix. These materials exhibit substantially
improved bond strength and exhibit no appreciable de-wetting, even upon
subsequent welding or brazing.
The following examples illustrate more clearly the manner in which mixed
boron-aluminum oxide interfaces of the present invention are formed. The
invention, however, should not be construed as being limited to the
particular embodiments set forth in the examples.
EXAMPLE 1
FP-alumina fibers, which are commercially available from DuPont, are vapor
deposited with a 2.5 micron thick layer of pure aluminum. The aluminum
deposition was conducted by physical vapor deposition. The fibers were
then heated above the melting point of aluminum or at 750.degree. C. for
15 minutes to provide melting of aluminum and allowed to cool.
FIG. 1 shows the strong de-wetting of aluminum metal on the alumina fibers
at 100X magnification. This phenomenon is evidenced by substantially all
of the aluminum metal being segregated into droplets on the surface of the
fibers. No appreciable spreading or bonding to the fibers of the aluminum
metal on the fibers is observed.
EXAMPLE 2
The procedure of Example is repeated except that prior to coating the
fibers with aluminum they are treated by immersion in a saturated aqueous
solution of ammonium pentaborate at 75.degree. C. for 10 minutes, dried in
ambient atmosphere, and subsequently heated in air at 1250.degree. C. for
1 hour.
FIG. 2 shows the ammonium pentaborate treated fibers at 100X magnification.
The treated fibers exhibited thermal decomposition of the pentaborate into
boron oxide forming an interoxidized layer of mixed oxides of B.sub.2
O.sub.3 and Al.sub.2 O.sub.3, including 2Al.sub.2 O.sub.3.B.sub.2 O.sub.3
and 9Al.sub.2 O.sub.3.2B.sub.2 O.sub.3, at the interface between the
fibers and the aluminum metal. FIG. 3 is a SEM of alumina fibers treated
with ammonium pentaborate, vapor coated with 2.5 microns of aluminum and
heated above the melting point of aluminum at 500X magnification. FIGS. 2
and 3 illustrate how the surface treatment increases the wetting of
alumina fibers by the molten metal. The aluminum metal wetted the fibers
with a substantially even homogeneous metal coating and produced very few
aluminum droplets on the surface of the fibers.
EXAMPLE 3
Two refractory alumina rods, 0.5 inch in diameter, are used to evaluate
interfacial bonding. The first is untreated and the second rod is treated
with a saturated ammonium pentaborate solution by immersing the rod
therein at 75.degree. C. for 10 minutes, drying the rod at ambient
conditions, and subsequently heating the treated rod at 1250.degree. C.
for 1 hour.
An aluminum matrix alloy of Al-4.5Cu-3 Mg was melted in a vacuum induction
furnace. The alumina rods are immersed in the liquid-phase aluminum alloy
and left therein until the surrounding alloy has undergone complete
solidification. The integrity and strength of the interfaces were
subsequently evaluated.
Load versus deflection data are collected with an Instron Testing Machine
at a cross head rate of 0.05 inches per minute. Examination and mechanical
testing of the specimens are reported below in Table I. Low bonding
occurred between the untreated alumina rod and the matrix alloy. However,
the treated rod demonstrated strong bonding at the interface between the
alumina and the matrix. Surprisingly, the treated rod produced an
interface that was ten (10) fold stronger than the bond between the
untreated rod and the matrix. The formation of mixed oxides permitted
effective wetting of the alloy to form a strong bond between it and the
alumina.
TABLE I
______________________________________
Bond Strength
Specimen (measured in Mpa)
______________________________________
treated alumina rod
7.98
and matrix
untreated alumina rod
.72
and matrix
______________________________________
EXAMPLE 4
A 400 .ANG. thick boron coating was vapor deposited onto FP-alumina fiber
surfaces. The boron-coated fibers and untreated fibers were each vapor
coated with a 2.5 micron thick layer of pure aluminum by physical vapor
deposition and heated until the aluminum melted.
FIG. 4 shows that mixed boron-aluminum oxide of the fiber/metal interface
permitted substantial and effective aluminum wetting as compared to that
permitted by the untreated fibers which exhibited no appreciable wetting.
EXAMPLE 5
An alumina rod, 0.5 inch in diameter, was introduced into a molten matrix
of Al-Cu-Mg-B having about 0.1 percent by weight boron (which is in excess
of the solubility limit for boron at the melting point of the matrix).
Solidification was permitted. Examination of the consolidated sample
showed substantially improved bonding at the interface between the alumina
rod and the boron-alloyed aluminum matrix.
Although the present invention has been described in terms of an alumina
fiber/aluminum matrix composite, other shapes of alumina may also benefit
from the present teachings. Thus for example, equiaxed and non-equiaxed
particulate alumina, planar alumina sheets and alumina whiskers, all
treated in accordance with the present invention, may be used to reinforce
aluminum alloys.
While the invention has been described in terms of preferred embodiments,
the claims appended hereto are intended to encompass all embodiments which
fall within the spirit of the invention.
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